1. Field of the Invention
The present invention relates to a multi-vortex flowmeter endowed with both a function of a vortex flowmeter and a function of a thermal flowmeter.
2. Related Art
Vortex flowmeters and thermal flowmeters are used for measurement of a flow rate of a fluid to be measured flowing through a flow tube. As is well known in the art, the vortex flowmeter utilizes the fact that when a vortex generator is arranged in a fluid flow, the number of Karman vortexes generated by the vortex generator per unit time (vortex frequency) is in proportion to the flow rate regardless of whether the fluid is a gas or a liquid in a given range of Reynolds number, and this constant of proportionality is called the Strouhal number. Examples of a vortex detector include a thermal sensor, a distortion sensor, an optical sensor, a pressure sensor, and an ultrasonic wave sensor. Those are capable of detecting a thermal change, a change in lift, etc., due to vortexes. The vortex flowmeter is a simple flowmeter capable of measuring a flow rate without being affected by physical properties of the fluid to be measured, and is widely used for the flow rate measurement of gases and other fluids (see, for example, JP 2869054 B (page 3, FIG. 1)). On the other hand, the thermal flowmeter is equipped with a fluid temperature detecting sensor and a heating side temperature sensor, and is controlled such that the temperature of the heating side temperature sensor (flow velocity sensor (heater)), which is endowed with a function of a temperature sensor and a function of a heating sensor, exhibits a fixed difference in temperature with respect to temperature as measured by the fluid temperature detecting sensor. This is due to the fact that the quantity of heat taken by the heater when the fluid to be measured is caused to flow is related to the mass flow rate, and the mass flow rate is calculated from the heating electric power amount with respect to the heater (see, for example, JP 2004-12220 A (page 6, FIG. 4)).